In injection molding machines a reciprocating screw within a cylindrical barrel pushes or injects a plasticized melt through an orifice at the barrel end or nozzle. The screw is then retracted in the barrel while rotated to advance new plastic material through the screw flights into the barrel space forward of the screw whereupon the injection stroke occurs again.
The temperature of the plastic melt from the feed hopper where the plastic material enters the barrel to the barrel nozzle where the plastic is injected to the mold must be carefully controlled. The control of the temperature of the plastic melt is affected by a number of factors. For example, the composition of the melt itself, the speed of rotation of the screw (shear heat), the temperature of the melt as it leaves the plasticating hopper, the speed of the ram during injection, etc. One of the factors influencing the temperature of the melt is the barrel heat. This invention relates to a control and a process for controlling the barrel temperature. Since the barrel temperature is one of the factors affecting the melt temperature, the invention relates somewhat to the melt temperature. However, the invention is not directed to any control, per se, of the melt temperature. The invention permits better control of one of a number of different elements of an injection molding machine, all of which cumulatively seek to give the operator a precise control of all variables in the molding process, one variable being the temperature of the melt.
Typically, control of the barrel temperature of an injection molding machine is achieved by electric heater coils or bands circumscribing the barrel and extending over discrete lengths of the barrel from the feed hopper to the barrel nozzle. Typically, four zones of heater bands are used, although many machines use three bands and some small machines have only two heater bands. Insofar as the machine's control console is concerned, present day controls give the operator the option of setting the barrel temperature for each heater band or zone to whatever temperature is desired depending on the characteristics of the plastic being molded. It should be appreciated that the injection stroke can be very rapid. Thus while the process is a batch process, rapidness of the cycle is such that the heater bands are simply turned "on" by a duty cycle throughout the entire plastic run of the machine. Current will simply pass through the resistance heating elements for a time regulated by a duty cycle and then be shut-off for a regulated time period. Because of the time it takes for heat to pass by conduction throughout the barrel, the bands are not purposefully adjusted during each molding cycle. However, each molding cycle will inherently produce temperature disturbances in the zones. This is one of the reasons why injection molding machines have more complex heat transfer problems than other similar systems which have steady-state characteristics such as extrusion processes.
Early controls for the heater bands were simple thermostat-like temperature switches and variacs. Such controls offered a manual mode for setting temperature of the bands without a thermocouple. However, such controls could not sense the temperature of the barrel and lacked any type of control which could account for any variation in the temperature of the plastic melt.
The most common control in use today is a P.I.D. controller implemented within the injection molding machine's main controller or through external temperature control modules. The typical installation uses a thermocouple embedded in the wall of the barrel for each heat band or zone. The thermocouple generates a closed loop feedback signal fed back to the P.I.D. controller, which also receives the temperature set point signal and generates a command, or driving signal, controlling the heater band duty cycle. The P.I.D. closed loop, feedback control system is commonly used in the control industry to calculate terms proportional to the error term, its integral and derivative, which are summed to achieve the controller output. Through proper selection of the gain terms, it may be "tuned" to solve a particular control problem and is viewed as being very "robust" in handling disturbances. However, the P.I.D. controller is a low order controller operating in accordance with classic feedback control concepts. Its disturbance response, steady state errors and peak overshoot must be compromised to arrive at a good set of "tuning" gain values which inherently, cannot accurately predict nor correct for higher order dynamics such as are present in a distributed parameter problem like heat conduction. As a general principle, the P.I.D. controller does not take into account any information about the process it is controlling. It simply reacts to feedback in a classical sense to correct an error.
Developments in the control art applied to temperature control of injection molding machines have, until now, taken the form of auto tuning or conceivably adaptive auto tuning of the P.I.D. feedback loop to arrive at a better selection of the gain term used in the P.I.D. loop, thus making the P.I.D. more responsive, etc. Fundamentally, this approach is defective because P.I.D. controllers are single in/single out controllers. The feedback law employed in single in/single out controllers does not take into account any information from the surrounding zones. Therefore, the controller must wait until the temperature effects of the surrounding zones travel through the barrel and are sensed by the thermocouple to generate sufficient feedback error at the zone for the closed loop to detect and respond to the error. This inherent deficiency or failure to account for temperature effects upstream and downstream of the zone translate into numerous control deficiencies experienced by the injection molding machine. One specific example of a deficiency in the P.I.D. controller is nozzle temperature overshoot due to an increase in the temperature in the front zone. Still more basic, zone temperature overshoot and ramp time to reach desired temperatures cannot be controlled by a P.I.D. controller to the extent that a controller could function if its gain term factored into account the temperature effects of the surrounding heat zones before the time lag occurs when such temperature effects are actually sensed. A control which doesn't rely then on feedback concepts to record an event which has occurred, but instead, estimates the occurrence of the event inherently then provides a significant improvement over prior art feedback control technology.
In the general control prior art, state controllers based on mathematical models utilizing state variables are known. Where disturbances in the system can be measured, it is known to reduce or compensate for the disturbances by means of feed forward (as contrasted to feedback). Where the disturbances cannot be measured, it is known to predict the disturbance using measurable signals and an observer incorporated into mathematical models based on various analysis, such as statespace or input-output. While theoretical discussions of control principles can be found in any number of text references (several of which are incorporated by reference herein), the practical incorporation of such concepts in high speed injection molding machines involving, among other things, numerous disturbances (many of which are not directly measurable), variations in barrel geometry, heat conduction coefficients, etc. has heretofore prevented the use of state controllers in injection molding machines.